Patent GB-1433315-A (GB)
Title: Brake friction member having a lining wear sensing device
IPC: F16D66/02
Filed: 1974-08-08
Abstract: 1433315 Brake lining wear warning devices; testing for lining thickness BENDIX CORP 8 Aug 1974 [30 Aug 1973] 35029/74 Heading F2E [Also in Division G1] A brake friction member 14 has a support member 18 carrying a friction lining 16 for engagement with a member 12 to be braked and also carrying a lining wear sensing probe 26 including electrically conductive material 38 which extends through the lining 16 from its surface adapted for engagement with the member 12 towards its surface adjacent to the support member 18 whereby as the lining 16 wears the material 38 wears progressively reducing its resistance and such resistance is measurable by an external test instrument 86 (Fig. 8) to determine the amount of lining remaining. With the friction member 14 in use on a vehicle sufficient wear of the material 38 will eventually lower its resistance to a point where a warning light 82 in an electrical circuit (see Fig. 7) is energized to warn the driver. The friction member 14 may comprise a disc brake pad or drum brake shoe of which the lining 16 is secured to the support member 18 by at least one rivet 20. As seen in Fig. 1 the probe has a plastic housing 28 with a cup-shaped portion 30 embedded in the lining 16 and a projection 32 which extends through an opening 34 in the support member 18 to be secured by a gripping ring 36. The conductive material 38 fills the cup-shaped portion 30. Such material 38 has high resistivity and carbon compounds may be used-preferably the carbon compounds are mixed with lining material so that the material 38 has similar thermal expansion characteristics to lining 16. A metallic conductor 42 connected to the warning light circuit has one end 44 in electrical contact with the material 38-the end 44 extends into the material 38 a distance slightly greater than a head 22 of the rivet 20 into the lining 16. With the embodiment of Fig. 1, the resistance of a circuit path through the material 38 between the end 44 and the member 12 during braking will decrease linearly as the lining 16 wears. Conductor 42 is of relatively low resistivity and the resistance of the circuit path drops to a low value which remains constant when the material 38 wears sufficiently for end 44 to contact member 12, and this will cause energization of warning light 82. In further embodiments of the probe 26 the resistance of the circuit path does not decrease uniformly as the lining 16 wears. In the embodiment of Figure 3 (not shown) the cupshaped portion (30) is tapered so that the area of material (38) engaging the member (12) is constantly decreasing to give an increasing rate of drop in resistance as the lining wears. In the embodiment of Fig. 5, the cup-shaped portion 26 is provided with sections 48, 50, 52 of relatively large cross-sectional area and sections 54, 56 of relatively small cross-sectional area filled with conductive material 38 to give a decrease in resistance incorporating "steps". A test instrument having a series of lights may be utilized with this embodiment, each additional light being actuated as the lining is worn by an additional "step". The circuit of Fig. 7 has probes 58, 60, 62, 64 for respective wheels of a vehicle connected in parallel to a connector 68 whose output connects with the base of a transistor 72 and through a resistor 74 to a voltage source +V. A transistor 80 is connected to the warning light 82 and to earth as at 76, and its base is connected to voltage source +V through resistors 78, 84. When all the linings are new the parallel resistances of probes 58-64 will be sufficiently high to maintain the potential at the base of transistor 72 at a relatively high value so that transistor 72 will be turned on so that the base of transistor 80 will be at a relatively low potential and the light 82 will be off. Should one of the linings wear much more rapidly than the others so that end 44 engages member 12 the resistance of the circuit path through the corresponding probe will be so low that the resistance of the other probes may be ignored. The base voltage on transistor 72 will therefore be lowered, turning the transistor 72 off and increasing the voltage on the base of transistor 80 to turn the latter on to actuate light 82. If all of the linings wear evenly, the parallel resistances of probes 58-64 will be low enough to turn off transistor 72 just before the ends 44 engage the corresponding members. In the instrument 86 of Fig. 8 for use in a service centre a switch element 106 of a switching mechanism 104 is connected to the respective probes in sequence and their resistances measured by means of a voltmeter 88 as an estimate of lining thickness (see Division G1).